1. Field of the Invention
The present invention relates to ethylene backbone copolymers, distillate oil compositions containing the copolymers, and an improved method of preparing the copolymers for use as pour depressants and flow improvers for distillate oil, which method reduces ethylene branching and involves free radical polymerization in a solvent, at temperatures below 130.degree.C. These copolymers comprise about 3 to 40 molar proportions of ethylene monomer per about one molar proportion of an unsaturated monomer other than ethylene, having number average molecular weights in the range of about 1,000 to 50,000 and preferably in the range of 1,000 to 2,900.
2. Description of the Prior Art
The commercially most important ethylene-containing pour depressant and flow improvers for distillate oil are copolymers of ethylene and ethylenically unsaturated ester monomers, such as the copolymers of ethylene and vinyl alcohol esters, particularly vinyl acetate, which copolymers are well known in the prior art. For example, U.S. Pat. No. 3,048,479 teaches copolymers of ethylene and C.sub.3 to C.sub.5 vinyl esters, e.g. vinyl acetate, having molecular weights of about 1,000 to 3,000 according to K. Rast's method of determining mol. wt. (Ber. 55, 1051, 3727 (1922)), as pour depressants for fuels, specifically heating oils, diesel and jet fuels. The copolymers of the examples of said patent were prepared by free radical catalysis, using ditertiary butyl peroxide as the catalyst, (although the patent teaches any peroxide catalyst), at temperatures of 280.degree. to 340.degree.F., in a solvent. U.S. Pat. No. 3,087,894 teaches a low cold test fogging oil containing as a pour depressant an ethylene-vinyl acetate copolymer essentially of the same type described in said U.S. Pat. No. 3,048,479. U.S. Pat. No. 3,131,168 teaches a free radical process for making ethylene-vinyl acetate copolymers as pour depressants for middle distillate using temperatures up to 440.degree.F., a solvent such as toluene or hexane, any peroxy compound or catalyst, but preferably ditertiary butyl peroxide, and adding additional ethylene to the reaction during the polymerization. U.S. Pat. No. 3,093,623 teaches still another method for making these ethylene-vinyl acetate pour depressants for middle distillates by continuously adding vinyl acetate and ethylene during the course of the reaction. In addition, U.S. Pat. No. 3,250,714 teaches ethylene-vinyl acetate copolymers having molecular weights of 3500 to 7000 as V.I. improvers for lubricating oils. However, all the aforesaid prior art patents on pour depressants are directed to carrying out the free radical polymerization at relatively high temperatures, i.e. in excess of 130.degree.C.
Now, generally speaking, the free radical polymerization temperature is related to: the decomposition temperature of the catalyst, i.e. the temperature has to be such to decompose the catalyst; the half life of the catalyst, i.e. the temperature cannot be so high that the catalyst decomposes so fast that it is used up before the reaction is completed; and the yield of polymer per pound of catalyst, i.e. catalyst utilization. In the aforesaid prior patents on peroxide catalyzed ethylene-unsaturated monomer copolymers as pour depressants, relatively high temperatures were used for the polymerization and catalyst is added continuously during the reaction. Thus, in general, the higher the temperature, the greater the activity of the monomers, which in turn gives a faster reaction time and a greater yield of polymer per pound of catalyst so that the reaction is more efficient. As a result, the incentive has been to use the peroxide catalysts at high temperatures. The method of the present invention differs from the prior art in deliberately operating at polymerization temperatures below the optimum for monomer utilization, and preferably using free radical catalysts with low decomposition temperatures. By this low temperature technique, it was found that the amount of ethylene branching is considerably reduced and copolymers produced by this method were superior pour point depressant and flow improvers to prior art copolymers prepared at higher temperatures. Specifically, the prior ethylene-vinyl acetate pour point depressants, while very effective in treating distillate fuel oil to lower the pour point, frequently results in wax crystals having large particle sizes ranging from one millimeter up to an inch in their largest dimension, depending upon the exact nature of the distillate oil, e.g. crude source, narrowness of the boiling range, etc. While the treated distillate oil containing these large wax crystals exhibits a pour point significantly under the original pour point of the untreated oil, in many cases, the large wax crystals will tend to plug filter equipment and lines normally used on delivery trucks and fuel oil storage systems when the oil is cooled below its cloud point, even though above its pour point. Thus, as the oil containing the pour point depressant is cooled, the cloud point (the point at which the oil becomes cloudy due to crystallization of wax) will generally be reached at a temperature significantly above the pour point (the point at which the oil can no longer conveniently be poured). As a result, oils below their cloud point and above their pour point will be pourable, but at the same time the wax crystals that have formed, if too large, can result in plugging the aforesaid filter equipment. Copolymerizing ethylene and vinyl acetate at a low temperature by the process of the invention can give the very good pour point reduction of the prior art ethylene-vinyl acetate copolymers, and in addition form smaller wax crystals during cooling of the treated oil. As a result, copolymers prepared by the method of the present invention, can regulate the maximum size of the wax crystals that are formed, on cooling at rates normally encountered during cold weather, to a particle size in the order of about 0.1 millimeter or less. Because of this, even though the cloud point of the oil treated with copolymer prepared by this new method is substantially the same as the cloud point when the oil is treated with the copolymer prepared by the older prior art methods, an important improvement in filterability and flow is obtained in actual operating use with elimination of clogged filters. It is not known exactly why these improvements occur. Nuclear magnetic resonance studies indicate that at lower polymerization temperature, the number of ethylene side branches forming on the main polymer chain is reduced, for example to 6 or less methyl terminating side branches per 100 methylene groups. And, for some reason, not clearly known, this reduced branching in ethylene-unsaturated monomer backbone type pour depressants appears to result in a more effective pour depressing and flow improving ability.